A typical electrical submersible pump (ESP) system 20 shown in FIG. 1 suspends from production tubing 16 in a wellbore casing 14 that penetrates a formation. A motor section 30 located on the bottom of the ESP system 20 operates a pump section 50. A seal section 40 positioned between the motor and pump sections 30 and 50 equalizes pressure in the motor section 30 with the external hydrostatic pressure. The pump section 50 connects to the production tubing 16 by a discharge head and has one or more pumps to lift fluid into the production tubing 16. The pumps in this section 50 can include centrifugal pumps, gear pumps, vane pumps, progressive cavity pumps, or the like and can have several stages.
To provide power and control to the motor section 30, a switchboard or variable-frequency drive (VFD) 22 at the surface connects to a junction box 24, and a surface cable 26 connects from the junction box 24 to the wellhead 12. The surface cable 26 then passes through a penetrator at the wellhead 12 and is now called an electric submersible pump (ESP) cable 28. The ESP cable 28 extends within the casing 14 down the length of the production tubing 16 and 28 is typically banded or strapped to the production tubing 16 at various intervals. Most of the length of the cable 28 has a round configuration. At some point near the pump section 50, the round ESP cable 28 is spliced by a splice component 29 to a flatter cable 70, typically called a motor lead extension (MLE) cable. The flatter MLE cable 70 is better suited to fit in the annulus between the pump section 30 and the casing 14 where the clearances can be very tight and where damage to the MLE cable 70 can occur. At a motor head 60, the MLE cable 70 connects to the head's pothead 62 and supplies electrical power to the motors in the motor section 30. Multiple motors may be stacked in tandem in the motor section 30 depending on the number of pump stages and the like used in the system 20.
In use, the VFD 22 receives power from a utility grid or generator (not shown). When the ESP system 20 is started, the motor section 30 draws the required voltage via the cables 28 and 70 and generates pump rotation. As the fluid comes into the well through the casing's perforations, the fluid passes the motor section 30 and the seal section 40 and enters the pump's intake 52. Inside, each pump stage (impeller/diffuser combination) adds pressure or head to the fluid at a given rate. Eventually, the fluid builds up enough pressure as it reaches the top of the pump section 50 so the fluid can be lifted to the surface through the production tubing 16.
As is known, the MLE cable 70 has conductors, insulation, barriers, jackets, armor, and the like, and these components can be arranged in different configurations depending on the implementation and the required power capacity. For example, the MLE cable 70 usually has a rigid outer jacket of stainless steel or galvanized steel armor that encloses several (e.g., three) inner cable legs. Typically, the MLE cable 70 has a flat configuration with the individual cable legs positioned side-by-side in the outer jacket, although other arrangements are also used.
Two or more electrical motors are typically connected in tandem to drive large pumps, and the motors can usually be three-phase, AC motors. The motors come in single, upper, center and lower tandem sections. Because the MLE cable 70 provides power to the ESP motors, failure in the cable 70 or its connection to the head 60 will prevent the ESP system 20 from operating properly (if at all). Therefore, it is important that the connection of the MLE cable 70 to the motor head 60 minimizes potential damage to the MLE cable 70 during installation and operation so the ESP system 20 will have optimum runlife.
In FIG. 2A, a typical upper tandem motor head 60A is shown for a motor section 30 of an ESP system. The head 60A connects to the seal section 40 above and connects to a tubular housing 32 of the motor section 30 below. A drive shaft 34 interconnected between the motor and seal sections 30/40 passes through the head's central passage 62. A guard sheath 63 separates portion of the shaft 34 from the interior of the motor section 30, and upper and lower bushings may be used to support the shaft 34 in the head 60A. Within the motor section 30, the shaft 34 has a rotor 38 positioned within a stator 36 of an electric motor, which may be a three-phase motor requiring three electrical conductors. The MLE cable 70 connects to the pothole 64 in the side of the head 60A via a pothead connector 72. From the pothead connector 72, internal conductors 74 then connect internally to the components of the motor section 30 to power the motors.
The connection of the MLE cable 70 to the upper tandem motor head 60A in FIG. 2A represents a typical configuration that uses the pothead 64 in the side of the motor head 60A for connection of the MLE cable 70. As an alternative to this configuration, the upper tandem motor head can be designed with a “pregnant” or “goitered” configuration to provide a direct-connect for the MLE cable 70.
FIG. 2B show an example of a “pregnant” or “goitered” upper tandem motor head 60B. As shown, the head 60B has many of the same components as the previous head 60A so that like reference numbers are used for the same components. The body of this pregnant head 60B is different, however. As shown, the head 60B has a bulging portion 66 with a channel 67. The MLE cable 70 connects with a direct connection 73 to the bulge's channel 67, and conductors 74 pass through the channel 67 to components of the motor section 30. Due to the required dimensions and physical characteristics of this type of head 60B, the head 60B has to be initially cast and then machined to obtain the finished part with the unique bulging portion 66 and channel 67. This type of head 60B must also be uniquely designed for a particular implementation, which increases the cost required to manufacture this more complicated type of motor head 60B.
What is needed is a way for virtually any type of upper tandem motor head (either new or used) to be easily converted into a direct-connect head for motor lead extensions so as to eliminate the cost of configuring a “unique” head for each implementation and to provide greater flexibility in meeting the needs of a given installation.